Method for joining aluminum power alloy

ABSTRACT

Sintered pieces of rapid-solidified aluminum alloy powder are friction stir welded together. The sintered piece may be composite material dispersing ceramic particle therein. A welding aid, which disperses the same ceramic particle as those in the sintered pieces, may be sandwiched between or mounted on the sintered pieces. A weld zone is formed without melting, so as to inhibit formation of blowholes or coarsening of metallographic structure. Consequently, the sintered pieces are welded together while retaining intrinsic properties of the sintered aluminum alloy.

FIELD OF THE INVENTION

The invention relates to a method of welding parts made of sinteredaluminum alloys or aluminum composite material.

BACKGROUND

Sintered aluminum alloys are manufactured by compacting and sinteringaluminum alloy powder. Various properties, e.g. strength,heat-resistance, abrasion-resistance, Young's modulus and low thermalexpansion coefficient, are imparted to the sintered aluminum alloys byselection and designing of alloying compositions and/or improvement ofprocessing, so that the sintered aluminum alloys are employed in variousfields. Especially, sintered alloy parts, which are made of aluminumalloy powder prepared by rapid solidification process, retain finemetallographic structures originated in the powder preparation, andexhibit good mechanical properties due to the fine metallographicstructures. Sintered alloy also has the advantage that ceramic particlescan be dispersed in a matrix with ease, although dispersion of ceramicparticles is difficult according to a conventional ingot process.Various properties, e.g. high strength, heat-resistance or neutronabsorption, are imparted to aluminum alloys by selection of ceramicparticles to be dispersed.

However, the powder metallurgy puts restrictions on profiles of sinteredalloy parts. Therefore, aluminum alloy pieces, which are compacted andsintered to tentative profiles, are welded together to fabricateobjective profiles suitable for use. Arc-welding, e.g. MIG or TIGwelding, is a representative method for welding the aluminum alloypieces.

For instance, sintered pieces, which are prepared by pressure sinteringaluminum powder mixed with ceramic particles for impartment of specialproperties, are heat-treated and then welded by a conventional weldingmethod, as disclosed JP 2002-504186T.

When aluminum alloy pieces are arc-welded together, large current shallbe supplied to the aluminum alloy pieces due to high electric andthermal conductivity of the aluminum alloy. Generation of heat duringwelding causes various defects, e.g. deformation derived from thermalstrain, reduction of strength at heat-affected zones or blowholes.Especially, sintered aluminum alloy parts occludes hydrogen therein at arate of 20-30 cc/100 g in an insufficiently degassed state. Theocclusion rate is very higher than a conventional casting (less than 1cc/100 g), resulting in formation of numerous blowholes during welding.Although hydrogen occlusion quantity is reduced by vacuum degassingprior to sintering or by sintering in a vacuum atmosphere, hydrogenstill remains in sintered alloy parts at a rate of 1-5 cc/100 g. As aresult, blowholes may be often formed due to the residual hydrogen. Ifthe degassing process is continued in a vacuum atmosphere for a longwhile, elements with low vapor pressure may be discharged from surfacesof alloy powder. The long-term heating also coarsens metallgraphicstructures. Moreover, parts to be welded are melted according to aconventional welding process, so that metallographic structures arecoarsened at the melted zones and the vicinities, resulting in reductionof strength at the weld zones compared with other parts. Furthermore,the coarsening cancels advantages of fine metallographic structuresoriginated in rapid-solidified alloy powder.

A welding process often uses filler material, for weldingdispersion-strengthened material comprising an aluminum alloy to whichceramic particles are dispersed. When such filler material does notdisperse ceramic particles as reinforcement therein, weld zones areformed in a reinforcement-free state and so weakened in comparison withother parts.

SUMMARY OF THE INVENTION

The present invention is accomplished to overcome the above-mentionedproblems, aiming at provision of welded bodies of sintered aluminumalloys without substantial difference in strength between weld zones andthe other parts.

The invention is characterized by friction stir welding sintered pieces,which are prepared by pressure sintering rapid-solidified aluminum alloypowder to a certain profile.

The sintered pieces may be composite material of quenched aluminum alloypowder with ceramic particle. The ceramic particle is preferably of 10μm or less in average particle diameter.

The friction stir welding process uses a welding tool, to which arotating pin of 3-10 mm in length and 3-10 mm in diameter is fixed,having a radius of shoulder within a range of 6-25 mm. The friction stirwelding is performed under conditions of: a rotation rate of the pinwithin a range of 500-3000 r.p.m., a travel speed of 200-1000 mm/minuteand a pushing depth of the shoulder within a range of 0-1 mm.

A welding aid may be inserted between or mounted on the sinteredaluminum alloy pieces to be welded. The welding aid is preferably madeof an aluminum alloy containing the same ceramic particle as in thesintered aluminum alloy pieces, but a ceramic-free aluminum alloy isalso used as the welding aid. When the sintered aluminum alloy piecesare welded together with a welding aid interposed therebetween, awelding aid with a T- or H-shaped section is preferably used in themanner that a vertical wall of the T-shaped section or a web of theH-shaped section is sandwiched between the sintered aluminum alloypieces.

A welding aid, which contains ceramic particle at different ratiosbetween a part to be sandwiched and the other parts, may be also used.

The inventive welding process is applicable not only for weldingsintered aluminum alloy pieces together but also for welding a sinteredaluminum alloy piece to an ingot aluminum alloy piece.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view of a friction stir welding process (citedfrom JP 9-508073T).

FIG. 2 is a view for explaining friction stir welding aluminum alloypieces together with a welding aid sandwiched therebetween.

FIG. 3 is a view for explaining friction stir welding aluminum alloypieces together with a welding aid mounted thereon.

FIG. 4 is a view for explaining friction stir welding aluminum alloypieces together with a welding aid having a T-shaped section.

FIG. 5 is a view for explaining friction stir welding aluminum alloypieces together with a welding aid having a H-shaped section.

FIG. 6A is a sectional view illustrating formation of a plastic regionduring friction stir welding using a welding aid having a T-shapedsection, and FIG. 6B is a sectional view of a weld joint aftercompletion of friction stir welding.

FIG. 7A is a microphotograph of a welded zone formed by friction stirwelding sintered pieces containing 5 mass % of B₄C of 9 μm in particlesize, and FIG. 7B is a microphotograph of a matrix of the same sinteredpiece, which is not affected by a weld heat.

FIG. 8 is a photograph of macrostructure of a welded part formed byfriction stir welding with a welding aid.

PREFERRED EMBODIMENTS OF THE INVENTION

A friction stir welding process uses a rotator 2 having a pin 24coaxially fixed to its top end, as shown in FIG. 1 (disclosed in JP9-508073 T). The rotator 2 is rotated and pressed onto workpieces 3 and4, so as to thrust the pin 24 into a matching department between theworkpieces 3 and 4. Matching department of the workpieces 3 and 4 areheated in frictional heat by the pin 24 and stirred by rotation of thepin 24. Metal at the matching departments of the workpieces 3 and 4which is plastically fluidized by the heat and stirring, is mixedbetween the workpieces 3 and 4. Since heat is rapidly diffused afterpassage of the pin 24, the metal is solidified and a weld zone 5 isformed between the workpieces 3 and 4. In FIG. 1, the numeral 22 is anupper part for connecting the rotator 2 to a driving source, the numeral23 is a shoulder for fixing the pin 24, and the numeral 1 is anon-consumable probe provided with the rotator 2.

In the friction stir welding, a weld zone is formed by plasticallyfluidizing and mixing metal with a friction heat and a strong stirringpower at matching departments of workpieces, but the metal is not meltedas noted in arc-welding. Therefore, the weld zone is not heated up to anexcessive high temperature, coarsening of metallographic structure orblowholes does not occur. Thus weld zone retains high mechanicalstrength.

Even when particle-dispersed composite pieces are friction stir welded,a weld zone is formed by mixing the composite material without insertionof a filler. As a result, the weld zone retains high mechanicalstrength, since it keeps a particle-dispersed matrix without thermaldeformation or blowholes.

In order to ensure plastic fluidization of metal in a weld zone, ceramicparticle mixed in composite material is preferably 10 μm or less inaverage particle diameter. If ceramic particle bigger than 10 μm isdispersed in the composite material, fluidization of metal in a weldzone becomes insufficient, resulting in heterogeneous distribution ofthe ceramic particle and strength reduction of a weld zone. The coarseceramic particle also abrades the rotating pin.

In case of friction stir welding composite material, which dispersesceramic particle or the like therein, welding conditions are determinedso as to form either a weld zone with high dispersoid concentration or aweld zone with low dispersoid concentration. The former weld zone issuitable for an increase of strength or neutron absorption, while thelatter weld zone is suitable for facilitation of welding withoutabrasion or damage of a rotating pin. When friction stir welding isperformed under the condition that a welding aid, made of an aluminumalloy containing ceramic particle at a controlled ratio, is sandwichedbetween or mounted on workpieces, a weld zone dispersing ceramicparticle at a ratio corresponding to the controlled ratio is formed.

Rapid-solidified aluminum alloy powder is preferably prepared by gasatomizing process. The aluminum alloy powder preferably has an averageparticle size of 20-100 μm. Fine alloy powder less than 20 μm isdifficult to manufacture and to handle due to poor fluidity. When coarsealloy powder above 100 μm is pressure sintered on the contrary, ametallographic structure of a sintered body is coarsened, resulting inpoor mechanical strength in spite of a sintered alloy.

The alloy powder is poured in an aluminum can or subjected to coldisostatic molding or spark plasma sintering for improvement of handling.In the case where a workpiece to be welded is composite material,ceramic particle is mixed to the alloy powder at this stage. The ceramicparticle to be mixed in the alloy powder is selected from the groupconsisting of Al₂O₃, ZrO₂, SiC, B₄C, WC, AlN, Si₃N₄, BN and TiB₂. Two ormore of ceramic particles may be mixed in the alloy powder. A mixingrate of ceramic particle is determined to aim for impartment of anobjective function.

The aluminum alloy powder, after being pre-treated for improvement ofhandling, is pressure sintered. The alloy powder may be subjected todegassing treatment such as vacuum suction in prior to pressuresintering. The alloy powder is preferably degassed while being heated,so as to accelerate removal of gases and to promote partial sinteringreaction. Concretely, the alloy powder is heated at a temperature higherthan 200° C. (preferably 450° C.) during degassing.

Pressure sintering may be hot working, e.g. extruding, forging orrolling, other than conventional sintering in a pressurized state. Suchmulti-stage process may be employed as hot-extruding or hot-rolling atfirst and then hot-forging.

Pressure sintered pieces provided as the above are then friction stirwelded. The sintered pieces may be heat-treated according to the purposein prior to or after the friction stir welding.

Friction stir welding process uses a welding tool having a radius ofshoulder within a range of 6-25 mm provided with a rotating pin of 3-10mm in length and 3-10 mm in diameter. Friction stir welding ispreferably performed under conditions of: a rotation rate of therotating pin within a range of 500-3000 r.p.m. at a travel speed of200-1000 mm/minute and a pushing depth of a rotary shoulder within arange of 0-1 mm.

A rotation rate above 3000 r.p.m. or a travel speed slower than 200mm/minute causes overheating and melting of parts to be welded,resulting in formation of coarse metallographic structure. On thecontrary, a rotation rate less than 500 r.p.m. or a travel speed above1000 mm/minute causes too much load applied to a rotator and breakdownof a rotating pin. A pushing depth of the rotator shoulder less than 0mm means that the rotator shoulder is not in contact with workpieces andleads to formation of a weld zone in an unstrained state. The unstrainedwelding allows plastic fluidization of metal over a broad rangeinappropriate for formation of a normal weld structure, resulting inpoor mechanical strength. If a pushing depth of the rotator shoulderexceeds 1 mm on the contrary, the rotating pin is often broken down dueto application of heavy load.

In order to enhance weld strength of a ceramic particle-dispersedsintered body, it is preferable to increase a ratio of ceramic particle,e.g. Al₂O₃, ZrO₂ or SiC, dispersed in a weld zone. In order to improveneutron absorption, it is preferable to increase a ratio of B₄Cdispersed in a weld zone. For the purpose, an aluminum alloy, whichdisperses ceramic particle therein at a relatively high ratio, isseparately prepared and shaped to a welding aid with a proper profile.The welding aid is sandwiched between or mounted on workpieces to befriction stir welded.

Concretely, a welding aid 6, made of a sintered aluminum alloycontaining ceramic particle, is sandwiched between workpieces 3 and 4(FIG. 2) or mounted on the workpieces 3 and 4 (FIG. 3), and theworkpieces 3 and 4 are friction stir welded by thrusting a rotating pin24 downwards into a space between the workpieces 3 and 4. However, analuminum alloy, which disperses ceramic particle therein at a ratio morethan ceramic particle in the workpieces 3 and 4, is not suitable for awelding aid, since excess inclusion of ceramic particle causes abrasionor breakdown of the rotating pin 24 or a shoulder 23.

A welding aid may have a T-shaped section (FIG. 4) or a H-shaped section(FIG. 5). A vertical wall of the T-shaped welding aid or a web of theH-shaped welding aid is sandwiched between the workpieces 3 and 4. Inthe case where the workpieces 3 and 4 are thick sintered pieces, theH-shaped welding aid is interposed between the workpieces 3 and 4, andfriction stir welding is repeated from upper and lower sides. TheH-shaped welding aid may be a divisible type, which is separated to eachpiece at a web of the H-shaped section, so as to enable insertion ofeach divided piece into a matching department between the workpieces 3and 4 from both sides.

FIG. 6A shows a plastic region W formed by a frictional heat derivedfrom rotation and travel of a rotating pin 24 during friction stirwelding using a T-shaped welding aid (FIG. 4), and FIG. 6B shows a weldzone 5 after completion of friction stir welding. Since the T-shapedwelding aid is used in the welding operation shown in FIGS. 4, 6A and6B, a shallow groove is formed at a center of the weld zone and a coupleof ridges are formed at both sides of the shallow groove, but any partof the weld zone is thicker than the original workpieces 3 and 4. Inshort, use of such a welding aid as to form ridged parts on the weldzone is preferable for assurance of weld strength, if the externalappearance does not cause any trouble. If the weld zone shall have aflat surface, the ridged parts can be removed by machining or grinding.

As for the T- or H-shaped welding aid, a dispersion ratio of ceramicparticle may be differentiated between a vertical wall of the T-shapedwelding aid or a web of the H-shaped welding aid, which is sandwichedbetween the workpieces, and a horizontal wall of the T-shaped weldingaid or a flange(s) of the H-shaped welding aid, which is mounted on theworkpiece. For instance, a content of ceramic particle is made greaterat the vertical wall of the T-shaped welding aid or the web of theH-shaped welding aid, but made smaller at the horizontal wall of theT-shaped welding aid or the flange(s) of the H-shaped welding aid. Whenfriction stir welding is performed using a welding aid with such adifferential content of ceramic particles, high weld strength is gainedwith less abrasion of a rotating pin or shoulder.

The other features of the invention will be clearly understood from thefollowing examples.

EXAMPLE 1

Each aluminum alloy of a composition in Table 1 was pulverized to 55 μmin average particle diameter by air atomization method.

The alloy powder was compacted to a cylindrical billet of 95 mm indiameter by cold isostatic molding with a pressure of 1200 kg/cm². Thebillet was degassed and sintered by 2 hours heating at 560° C. invacuum. After the sintered billet was cooled to a room temperature, itwas reheated to 500° C. by an induction heater and then extruded (i.e.,pressure sintered) to a flat sheet of 4 mm in thickness. Thereafter, thesheet was subjected to T6 treatment (solution heating at 520° C. for onehour, water quenching and then aging at 180° C. for 6 hours).

The heat-treated sheet was friction stir welded together, using awelding tool having a radius of shoulder of 12 mm provided with arotating pin of 5 mm in length and 4 mm in diameter under conditions of:a rotation rate of 1500 r.p.m., a travel speed of 400 mm/minute and apushing depth of 0.5 mm.

Test pieces, which involved a weld zone, were sampled from the weldedsheet and subjected to a tensile test. Test results are shown in Table2.

For comparison, extruded parts with C-shaped section were MIG weldedtogether, using a filler JIS A4043. Test pieces, which involved a weldzone, were sampled from the welded body and subjected to the sametensile test. Test results are also shown together in Table 2. TABLE 1Chemical Composition of Samples (mass %) Alloy No. Si Fe Mg Cu Cr Sm Gd1 0.6 0.2 0.8 0.3 0.2 — — 2 0.8 4.8 0.9 0.2 0.3 — — 3 2.0 2.0 2.5 1.0 —— 2.0 4 2.0 2.0 2.5 1.5 — 5.3 — 5 25 0.2 0.9 0.2 0.3 — —

TABLE 2 Welding Method and Mechanical Properties Tensile Yield AlloyWelding Strength Strength Elongation No. Method (MPa) (MPa) (%)Inventive Examples 1 FSW 380 367 10.6 2 FSW 412 387 8.7 3 FSW 369 3425.8 4 FSW 325 309 4.9 5 FSW 381 341 0.7 Comparative Examples 1 MIG 121immeasurable immeasurable 2 MIG  89 immeasurable immeasurable 3 MIG  79immeasurable immeasurable 4 MIG 111 immeasurable immeasurable 5 MIGimmeasurable immeasurable immeasurableFSW: Friction stir weldingMIG: MIG welding

Results in Table 2 prove that the inventive examples, i.e. friction stirwelded pieces, had high strength.

On the other hand, the comparative examples, i.e. MIG welded pieces,were extremely inferior in mechanical strength and elongation, so thatthe tensile test itself was difficult. The inferior properties arecaused by formation of blowholes and coarsening of metallographicstructure. In fact, when the MIG welded pieces were inspected byultrasonic reflect scope, many defects, probably derived from blowholes, were detected.

When the friction stir weld zone was inspected by ultrasonic reflectscope, no defects were detected. A fracture surface formed by thetensile test was a normal ductile fracture plane.

EXAMPLE 2

Several powdery compositions were prepared by mixing aluminum alloypowders Nos. 2 and 3 in Table 1 with ceramic particles, Al₂O₃, SiC, B₄Cand AlN, of average particle diameters at mixing ratios in Table 3. Eachcomposition was compacted, extruded and welded by the same way asExample 1. The welded peaces were subjected to the same tensile test asExample 1.

Results are also shown in Table 3. TABLE 3 Mixing Ratios of CeramicParticles and Mechanical Properties Ceramic Particles Mixing ParticleRatio Weld- Tensile Yield Elon- Alloy size (mass ing Strength Strengthgation No. Kind (μm) %) Method (MPa) (MPa) (%) Inventive Examples 2Al₂O₃ 8 8 FSW 440 420 6.5 2 Al₂O₃ 20 8 FSW 410 380 4.3 2 SiC 8 15 FSW480 455 2.0 3 B₄C 9 5 FSW 435 412 4.8 3 AlN 8 5 FSW 398 374 6.0Comparative Examples 2 Al₂O₃ 8 8 MIG 165 — — 2 SiC 15 15 MIG 140 — — 3B₄C 9 5 MIG 280 130 1.5 3 AlN 8 5 MIG 175 110 1.0FSW : Friction stir weldingMEG : MEG welding

It is noted from Table 3 that the inventive examples, i.e. friction stirwelded pieces, had high tensile strength even at weld zones. The weldedpiece, made of the alloy No. 1 dispersing ceramic particle of 10 μm orless in size, had higher strength than the welded piece, made of thealloy No. 2 dispersing ceramic particle above 10 μm in size. Moreover,it was recognized that a rotating pin, which was used for friction stirwelding the alloy No. 2 dispersing ceramic particle above 10 μm in size,was heavily abraded in comparison with other examples.

On the other hand, the comparative examples, i.e. MIG welded pieces,were extremely inferior in mechanical strength and elongation, so thatthe tensile test itself was difficult. The inferior properties arecaused by formation of blowholes and coarsening of metallographicstructure. In fact, when the MIG welded pieces were inspected byultrasonic reflect scope, many defects, probably derived from blowholes, were detected. No ceramic particles were observed on fracturesurface.

When the inventive friction stir weld zone was inspected by ultrasonicreflect scope, no defects were detected. Observation results onmetallographic structures of welded zones and base metals with anoptical microscope prove that there were no substantial difference indispersion of ceramic particles between the weld zones and the basemetals. As an example, FIGS. 7A and 7B show microstructures of a weldedzone and a base metal, respectively, when a sintered piece, made of analloy No. 3 containing B₄C of 9 μm in size at a ratio of 5 mass %, wasfriction stir welded.

EXAMPLE 3

A powdery composition, prepared by mixing aluminum alloy powder No. 3(55 μm in average particle diameter) in Table 1 with B₄C powder (9 μm inaverage particle diameter) at a ratio of 5 mass %, was compacted andextruded by the same way as Example 1.

Two flat bars of 5.5 mm in thickness were provided in this way. Awelding aid was separately prepared, by extruding an aluminum alloy JIS6N01 to a T-shaped section of 1.0 mm in thickness and 18.0 mm in width.The welding aid was sandwiched between the flat bars as FIG. 4 andfriction stir welded under the same conditions as Example 1.

FIG. 8 is a photograph of a macrostructure of a weld zone. It is notedthat the flat bars were sufficiently welded together with the weldingaid.

INDUSTRIAL APPLICABILITY OF THE INVENTION

According to the present invention as above-mentioned, sintered piecesof rapid-solidified aluminum alloy powder are friction stir welded, anda weld zone is not melted so as to inhibit formation of blowholes andcoarsening of metallographic structure. As a result, the sintered alloypiece can be welded without any decrease of its original mechanicalproperties. Especially, composite material, dispersing ceramic particlesas reinforcement therein, can be welded according to the invention, soas to provide a welded structure, which retains an originalparticle-strengthening effect.

Therefore, application of sintered aluminum alloys and compositematerials is greatly expanded to various uses.

1-8. (canceled)
 9. A method of welding a sintered aluminum alloy,characterized by friction stir welding sintered pieces prepared bypressure sintering rapid-solidified aluminum alloy powder.
 10. Thewelding method of claim 9, wherein the sintered pieces are compositematerial prepared by pressure sintering a mixture of rapid-solidifiedaluminum alloy powder with ceramic particle.
 11. The welding method ofclaim 10, wherein the ceramic particle has an average particle diameterof 10 μm or less.
 12. The welding method defined by claim 9, wherein thefriction stir welding is performed using a welding tool having a radiusof shoulder within a range of 6-25 mm provided with a rotating pin of3-10 mm in length and 3-10 mm in diameter under conditions of: arotation rate of the rotating pin within a range of 500-3000 r.p.m., atravel speed within a range of 200-1000 mm/minute and a pushing depth ofa rotator shoulder within a range of 0-1 mm.
 13. The welding methoddefined by claim 10, wherein a welding aid, made of an aluminum alloydispersing the same ceramic particle as in the sintered piece, issandwiched between or mounted on the sintered pieces, and friction stirwelded together with the sintered pieces.
 14. The welding method definedby claim 10, wherein the sintered pieces are friction stir weldedtogether with a welding aid, made of an aluminum alloy free of ceramicparticle, being sandwiched between or mounted on the sintered pieces.15. The welding method of claim 13, wherein the welding aid has a T- orH-shaped section, a vertical wall of the T-shaped section or a web ofthe H-shaped section being sandwiched between the sintered pieces. 16.The welding method of claim 15, wherein the welding aid comprises afirst part to be sandwiched between the sintered pieces and another partnot to be sandwiched between the sintered pieces, the first part havinga ratio of ceramic particles different from the other part.
 17. Thewelding method of claim 14, wherein the welding aid has a T- or H-shapedsection, a vertical wall of the T-shaped section or a web of theH-shaped section being sandwiched between the sintered pieces.
 18. Thewelding method of claim 17, wherein the welding aid comprises a firstpart to be sandwiched between the sintered pieces and another part notto be sandwiched between the sintered pieces, the first part having aratio of ceramic particles different from the other part.